Mud pumps utilized to pump drilling mud at high pressures such as about 2,000 psi, are subject to rapid wear. A commonly utilized type of piston assembly includes a steel rod which supports a hollow piston member formed of rubber or felt, to reciprocate in a steel cylinder. The piston member wears rapidly and must be replaced at frequent intervals. After some wear of the piston member, high pressure mud can squirt around it and past a steel shoulder on the piston rod which abuts one end of the piston member. To avoid costly damage to the shoulder, the shoulder is formed with a replaceable wear ring which is the first portion to wear and which prevents a portion of the shoulder which is integral with the rest of the piston from becoming worn.
The useful life of a piston member can be extended by forming the member so that it can expand somewhat to seal against the cylinder even after considerable wear. This can be accomplished by forming the cylinder member with a concave face which faces the high pressure mud being pumped, so that the mud tends to expand the cylinder radially outwardly. However, this construction of the piston face can also tend to cause pressured mud to be forced between the inside of the hollow piston member and the piston rod. Such leakage of pressured mud can be avoided by forming the piston member with a complex shape or by utilizing a very thick piston member and a large protective end plate. However, it would be desirable if a relatively narrow-walled piston member of relatively simple shape could be utilized, so that replaceable piston members could be constructed at low cost, especially if advantage is to be taken of better wearing but higher cost materials.